1. Field of the Invention
The present invention relates to a wet type flue-gas desulfurization apparatus and a method for adjusting the oxidation reduction potential of an absorbent in the wet type flue-gas desulfurization apparatus.
2. Description of the Related Art
Regarding a flue-gas desulfurization apparatus that removes the sulfur oxide present in the flue gas which is generated as a result of burning fuel oil or coal in a thermal power station; a wet type flue-gas desulfurization apparatus is widely prevalent in which the flue gas and an absorbent slurry (formed of a calcium compound such as limestone) are placed in contact with each other in an absorber so that the sulfur oxide present in the flue gas is absorbed in the absorbent slurry, and the post-contact absorbent slurry is oxidized and subjected to solid-liquid separation to produce gypsum as a by-product.
In this case, the sulfur dioxide that is the principal sulfur oxide present in the flue gas is absorbed in the absorbent according to the reaction formulae given below, and reacts with the oxygen present in the flue gas or the oxygen supplied from outside to produce gypsum.SO2+H2O→H++HSO3−  (1)H++HSO3−+½O2→2H++SO42−  (2)2H++CaCO3+SO42−→CaSO4+CO2+H2O  (3)
In such a wet type flue-gas desulfurization apparatus, the oxidation reduction potential (ORP) of the absorbent is controlled using the oxidation air volume. Hence, in an absorbent accumulating unit, normal air (oxygen) is supplied and the oxidation air volume is adjusted.
However, for example, depending on the boiler combustion state, there are times when the oxygen (O2) in the flue gas increases in concentration than the expected level or when the sulfur oxide (SO2) decreases in concentration than the expected level. In such cases, even if the oxidation air volume that is introduced in the absorbent accumulating unit of the desulfurization apparatus is reduced to zero, the natural oxidation occurring due to the contact between the flue gas and the absorbent present inside the absorber leads to substantial oxidation of the sulfurous acid that is produced as a result of absorbing of the sulfur oxide. Besides, the absorbent falls in a peroxidative state. As a result, it becomes not possible to control the ORP at a desired level. For example, consider a case in which the design is meant to control the desired ORP value at 100 mV but the ORP value unstably hovers at an extremely high value such as a value between 200 mV to 1000 mV thereby leading to excessive oxidation. In that case, for example, manganese oxide is produced thereby leading to issues such as gypsum coloration, pH meter malfunctioning due to scaling, nozzle blockage, and gypsum dehydrator clogging. Moreover, the selenium present in the absorbent changes from the quadrivalent state to the hexavalent state, thereby making its removal difficult. Furthermore, the effluent treatment standards cannot be maintained due to the purification of persulfuric acid, thereby making separate aftertreatment necessary.
In that regard, conventionally, there has been a proposal to measure the oxidation reduction potential of the absorbent using an ORP meter; to adjust the supply of a gas containing oxygen according to the oxidation reduction potential; and, if the oxidation reduction potential increases to exceed the adjustable range of supplying the gas containing oxygen, to supply an oxidation inhibitor to the absorbent with the aim of adjusting the oxidation reduction potential (Patent Literature 1, Japanese Laid-open Patent Publication No. 2003-340238).
However, in the case of supplying an oxidation inhibitor as proposed in Patent Literature 1, it is difficult to control the dosage of the oxidation inhibitor. That is because, every time the oxidation inhibitor is added, although there is a decrease in the electric potential of the ORP, the addition conditions change every time in a subtle way depending on the boiler combustion conditions and the oxygen concentration conditions in the flue gas. Besides, once the oxidation inhibitor is added, the ORP value undergoes a drastic change, thereby making it difficult to control the ORP value at the desired value.
In that regard, there has been a demand for a wet type flue-gas desulfurization apparatus in which, for example, even if the oxidation air volume supplied in the absorbent accumulating unit is reduced to zero, the ORP can be controlled at the set ORP value as desired.